The GM 4T65eHD design was changed by GM in 2003. The electronic signals that shift the later transmissions (03-up) are different and the 95-02 PCM's cannot shift them. . The 2003 PCM can be used to control your powertrain combination. It is the only GTP PCM that can control a cable operated TB 3800SC and shift the later transmission. Your harness must be based on the 2003 Grand Prix 3800 engine. AShould you not want to build that harness yourself; a number of forum members here can build that harness for you.
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite.
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "